I think this is another X-Y question..but anyway,
is the last row in page 201 an existing record in the DB?
would you consider alter the design of p201 have separate reagions of the readonly and editable rows? easier this way.
if not, you got 2 options, i think.
1, build manual tabular forms (look into APEX_ITEM package) and make rows readonly from the source(give them readonly html attributes), CASE on the P200 condition.
I know this is not a real coded solution, but hopfully get you thinking of the solution/design a bit more before fixed into the coding.
if you want to all rows of tabular form in readonly mode except of new row created
then create dynamic action on page 201 like below:
Event: Page Load
Condition: No Condition
Fire on Page Load : Unchecked
selection Type : Region
Region: Select Region name
Hope This Helps...